A solid oxide fuel cell (SOFC) is a very promising energy conversion device to convert fuel chemical energy directly to electricity as schematically shown in FIG. 1. In addition to high energy conversion efficiency and low emissions, another significant advantage of SOFC is fuel flexibility. Fuels like hydrogen, natural gas, propane, biogases, liquid hydrocarbons or coal can be used as SOFC fuels.
However, there are two major challenges for a hydrocarbon fueled SOFC. One challenge is carbon deposition at the anode/electrolyte interface, which can lead to rapid degradation of the cell performance. Another challenge is sulfur poisoning which can dramatically degrade SOFC performance even when sulfur species are presented at the ppm level. Sulfur species such as hydrogen sulfide (H2S) are widely present as impurities or additives in many economically available fuel sources, and their concentration can reach high levels as shown in Table 1.
External reforming of hydrocarbon fuels and removal of low concentration sulfur (at the ppm level) can add extra cost to the SOFC system. In order to prevent or mitigate coking deactivation of the SOFC anode for direct utilization of hydrocarbon fuels, three different strategies have typically been applied. One strategy is to develop new anode systems, such as Cu-based cermets and perovskite oxide anode such as La0.75Sr0.25Cr0.5Mn0.5O3−δ (LSCM) and Sr2MgMoO6−δ. Another strategy is to infiltrate noble metal catalyst (such as Pd and Ru) or alloy electrocatalysts (such as Sn/Ni alloy) into porous anode support. A third strategy is to fabricate a catalytic barrier on the cermet anode for internal reforming
TABLE 1Typical composition and H2S concentration of some fuel sourcesFuel typeTypical compositionH2S concentrationCoal syngasH2, CO, CO2, H2O, N2100-300 ppmBiogasH2, CO, CO2, CH4, H2O, N2 50-200 ppmNatural gasH2, CO2, N2, C2H6, CH4>1%
Mechanistic investigation of sulfur poisoning of the SOFC anode has received increasing research interest recently since better understanding of this issue is crucial for identifying specific mitigation solutions against degradation, including development of sulfur-tolerant anodes. Typically, the performance loss of SOFC anodes in sulfur-containing fuels can be attributed to: (1) physical adsorption/chemisorption of H2S at surface active sites, leading to reduction of surface area for electrochemical reactions, and (2) sulfidation of anode material due to reaction between sulfur and anode materials resulting in loss of catalytic activity, conductivity and stability. Until now, sulfur-tolerant anodes have been categorized into three types of materials: (1) thiospinels and metal sulfides such as NiFe2S4, WS2 and CuCo2S4, (2) cermets such as Ni-scandia-doped zirconia oxide (SSZ) and Cu-ceria anode, and (3) mixed ionic and electronic conductors (MIECs) such as Ti-based perovskite anodes and lanthanum vanadate (La1xSrxVO3). Recently, surface Sb/Ni alloys were found to efficiently minimize the negative effects of sulfur on the performance of Ni/zirconia anode-supported solid oxide fuel cells (SOFCs).
Perovskites with both ionic and electronic conductivity at high temperature and in a reducing environment have received increasing interest in recent years on their application as SOFC anodes or anode components due to reduced interfacial polarization resistance by expanding reaction sites to the whole anode, relatively good compatibility with high-quality electrolytes, mechanical stability during long term service without expansion of metal components, relatively good catalytic activity for hydrogen and hydrocarbon fuel, and higher sulfur tolerance compared to NiO—YSZ components. Therefore, many efforts have been devoted to develop various mixed-ionic-electronic conductor (MIEC) anode materials for the application on a fuel-flexible SOFC with sulfur tolerance.
Although some perovskite materials have shown good catalytic activity for hydrocarbon fuel and have been examined for application as sulfur-tolerant anodes, few reported perovskite anodes can simultaneously display both good sulfur tolerance and catalytic activity for hydrogen and hydrocarbon oxidation.